This section provides background information related to the present disclosure which is not necessarily prior art.
A 3-wire transformer-free uninterruptible power supply (“UPS”) system is a 3 phase system without input and output transformers with the 3-wires all being hot. This type of UPS system is typically designed so that the DC voltage on its DC bus is as low as possible so that lower cost semiconductor devices and filter capacitors can be used. This choice, however, as is known in the art, requires that the type of pulse width modulation (“PWM”) control used to control the inverter of the UPS system include common mode voltage injection so that the inverter produces at its output the desired peak AC output line-to-line voltage.
During normal on-line modes of operation (when the inverter is not paralleled with the bypass power source), this common mode voltage is not seen by the load or the utility. However, in 3-wire transformer-free UPS systems for which separate power sources for the rectifier and bypass are utilized, when the inverter of the UPS is paralleled with the bypass power source, this common mode voltage can drive a common mode current. This common mode current gets larger if the rectifier power source is not in sync with the bypass source. To alleviate this problem, a common mode choke is inserted between the rectifier power source and the rectifier that reduces the common mode current. This common mode choke is basically an inductor that is expensive, takes up a fair amount of space and is very heavy.
FIG. 1 is a basic block diagram of a of a typical a prior art 3-wire transformer-free double conversion UPS system 100 where the power sources to which the rectifier and bypass switch are coupled are separate power sources. UPS system 100 includes a common mode choke 102, a rectifier input inductor 104, a rectifier input capacitor 106, a three phase rectifier 108, a DC bus 110, a three phase inverter 112, an inverter output inductor 114, an inverter output capacitor 116, a battery (not shown), a bypass switch 120, and a control module 122. DC bus 110 is a split DC bus having a high rail 124, a mid-point rail 126 and a low rail 128 that are coupled to corresponding power outputs of rectifier 108 and power inputs of inverter 112. An input 130 of rectifier 108 is coupled through rectifier input inductor 104 and common mode choke 102 to a rectifier power source 132. Rectifier input capacitor 106 is coupled between a junction of common mode choke 102 and rectifier input inductor 104 and a common 134 (which floats about ground but is not directly connected to ground). An output 133 of inverter 112 is coupled through inverter output inductor 114 through an over current protection device 136 (such as a fuse) to load 138. Inverter output capacitor 116 is coupled between a junction of inverter output inductor 114 and over current protection device 136 and common 134. Control module 122 is coupled to bypass switch 120, rectifier 108 and inverter 112. It should be understood that these power sources are not part of UPS system 100 but are typically power feeds such as from a utility.
FIG. 2 is a basic schematic showing the double conversion portion of UPS system 100 in more detail. As can be seen from FIG. 2, each of the three phases (A, B and C) includes a respective common mode choke 102 (designated 102A, 102B, 102C), a respective rectifier input inductor 104 (designated 104A, 104B, 104C), a respective rectifier input capacitor 106 (designated 106A, 106B, 106C), a respective inverter output inductor 114 (designated 114A, 114B, 114C), a respective inverter output capacitor 116 (designated 116A, 16B, 116C) and an over current protection device 136 (designated 136A, 136B, 136). The three phases of three phase rectifier 108 are designated 108A, 108B, 108C) and the three phases of three phase inverter 112 are designated 112A, 112B, 112C. A respective input 130 (designated 130A, 130B, 130C) of each rectifier phase is coupled to a respective phase (designated 132A, 132B, 132C) of rectifier power source 132. DC bus 110, comprised of high rail 124, mid-point rail 126 and low rail 128, is a common DC bus for the three phases.
When UPS system 100 is operating in a double conversion mode, rectifier 108 converts the incoming AC power from rectifier power source 132 to direct current (“DC”) power on DC bus 110. Inverter 112 converts the DC power on DC bus 110 to AC output power that is then supplied to load 138. In some cases, UPS system 100 is switched to a bypass mode where bypass switch 120 is closed. AC power then flows directly from a bypass power source 140 to load 138. UPS system 100 may be switched to the bypass mode when a component of UPS system 100 in the double conversion power flow path fails, such as rectifier 108 or inverter 112, or when the quality of AC power from bypass power source 140 is sufficient that it can be used to directly power load 138 without being conditioned through the double conversion path. It should be understood that power sources 132, 140 are not part of UPS system 100 but are typically power feeds such as from a utility.
Control module 122 controls the rectifier 108, inverter 112, and bypass switch 120. Control module 122 monitors the input and output voltages and currents and controls the rectifier 108 to charge the battery (not shown) and regulate the DC power including the DC bus voltage and also controls inverter 112 to regulate the AC output power including the AC voltage.
The output voltage of UPS system 100 is generated by control module 122 modulating the pulse width of the output of inverter 112. It typically does so by switching power semiconductors (FIG. 2) of inverter 112 at the desired modulated pulse width.
Under normal operating conditions the inverter 112 synchronizes the output voltage to the voltage of bypass power source 140. Since the rectifier power source and bypass power source are different power sources, inverter 112 and rectifier 108 may not be in sync. With the inverter 112 and rectifier 108 not in sync, when the inverter 112 is paralleled with the bypass power source 140, there will be a common mode current induced due to the PWM method used to control inverter 112, which is implemented in control module 122. In FIG. 1, the common mode current path is designated by the dotted line 142. The load current path when UPS system 100 is in the bypass mode is designated by the solid line 144.
To limit this common mode current, common mode choke 102 is inserted in the common mode current path. The dimensions and weight of the common mode choke depend on the power rating of the UPS. For example, a common mode choke for a 800 kVA UPS could have a volume of 3500 in3 and weigh over 500 pounds. The bulk of the weight is in the copper windings and laminated steel in the core which could cost as much as $2,500. So if this common mode choke could be eliminated, the UPS would cost less and have a smaller footprint.